Make image decoders faster by refactoring hot loops that fills the lines of ImageFrame.

Source/wtf/MathExtras.h

Index: Source/wtf/MathExtras.h
diff --git a/Source/wtf/MathExtras.h b/Source/wtf/MathExtras.h
index fa62ebb6c98ba75845e97a11923abc1332ff6d59..4ec682e8a6855f38cd4c8f28f99d2e348b25ee9d 100644
--- a/Source/wtf/MathExtras.h
+++ b/Source/wtf/MathExtras.h
@@ -434,6 +434,14 @@ inline unsigned fastLog2(unsigned i)
     return log2;
 }
 
+inline uint16_t fastDivideBy255(uint16_t value)
+{
+    // This is an approximate algorithm for division by 255, but it gives accurate results for 16bit values.
+    uint16_t approximation = value >> 8;
+    uint16_t remainder = value - (approximation * 255) + 1;
+    return approximation + (remainder >> 8);
+}
+
 } // namespace WTF
 
 #endif // #ifndef WTF_MathExtras_h